1. Field of the Invention
The present invention relates to a scanning exposure method, in which the projection region of a mask is limited by a slit and the entire pattern region of the mask is transferred onto a substrate by scanning the slit relative to the mask and the substrate, particularly, to a scanning exposure method for multiple exposure of the pattern of the mask on the substrate.
2. Description of the Related Art
In recent years, a method of effectively increasing the depth of focus by applying a multiple exposure is proposed in the projection exposure technology for transferring the pattern on a mask onto, for example, a semiconductor substrate. For example, proposed in Japanese Patent Publication (Kokoku) No. 4-10209 is a multiple exposure in which a mask, an optical projection system, etc. are moved in the direction of optical axis during the exposure so as to suppress the fluctuation in the pattern size. On the other hand, Japanese Patent Publication (Kokoku) No. 5-88531 discloses the idea of effectively increasing the depth of focus of an optical exposure system by superposing a plurality of light beams having different imaging points on the same optical axis.
The conventional multiple exposure method pointed out above, which can certainly be applied to a step-and-repeat type exposure apparatus, cannot be applied as it is to a so-called xe2x80x9cstep-and-scanxe2x80x9d type exposure apparatus, i.e., a scanning exposure apparatus in which the mask and the substrate are moved in parallel relative to the optical projection system.
In the scanning exposure apparatus, a mask is scanned in a one dimensional direction and, at the same time, a wafer is also scanned in a one dimensional direction in synchronism with the scanning of the mask. Under this condition, a slit-like exposure region is scanned on the mask surface so as to transfer the entire pattern region of the mask onto a transfer region. What should be noted is that the scanning exposure apparatus is constructed such that the mask and the wafer are moved relative to each other in a direction perpendicular to the optical axis of the optical projection system. Therefore, if the wafer and the optical projection system are moved in the direction of the optical axis relative to each other during the exposure, focused portions and portions that are not focused are present together depending on the positions of the transfer region. It follows that it is difficult to expect the effect of increasing the depth of focus. As a matter of fact, the resolution of the image is lowered.
Under the circumstances, a new method, which makes it possible to effectively apply a multiple exposure method to the scanning exposure apparatus, is proposed in Japanese Patent Disclosure (Kokai) No. 11-214301. In the new technology proposed in this prior art, the optimum imaging plane and the surface of the exposed region on the substrate are scanned as required in an inclined fashion with respect to a one dimensional scanning direction when the mask and the substrate are moved in parallel relative to the optical projection system so as to perform a multiple exposure method in which the depth of focus is seemingly increased. Also, it is possible to obtain a uniform effect of increasing the depth of focus in the longitudinal direction (non-scanning direction) of the slit region by simply controlling the leveling (inclined) state and the focusing state interlocked with the movement of the substrate in a one dimensional direction such that the relative inclination of the image plane within the slit region restricted within a circular imaging field of the optical projection system and the surface of the exposed region on the substrate is maintained within the range of the depth of focus of the optical projection system.
It is known to the art that it is difficult to set appropriately the exposure conditions in the multiple exposure method described above, though the multiple exposure method certainly produces a large effect in respect of the isolated pattern. It should be noted in this connection that, in the isolated pattern, the light intensity itself is markedly lowered with progress in the miniaturization of the pattern size and in the shift of focus so as to make the resolution difficult. To be more specific, the influence given by the reduction in the light intensity caused by the shift of focus is rendered prominent with reduction in the pattern size and in the image contrast so as to make it impossible to maintain a sufficient transfer accuracy.
A measure for overcoming the above-noted problem is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 7-153658. Specifically, it is disclosed that, in the process of performing a multiple exposure, the exposure is performed under the conditions which permit suppressing the reduction in the light intensity in the defocusing step and the exposure is performed under the conditions which permit ensuring a light intensity as high as possible in the center of the pattern in the focusing step so as to optimize the overall light intensity profile. The optimization can be achieved easily by changing the coherency factor "sgr" of the projection exposure apparatus.
The definition of the coherency factor "sgr" will now be described with reference to FIG. 1. The projection exposure apparatus shown in FIG. 1 comprises an illumination 1, a condenser lens 2, a photomask 3, a projection lens 4 and a silicon wafer 5. As shown in FIG. 1, the numerical aperture NAC of the condenser lens 2 and the numerical aperture NAp of the projection lens 4 are given as: NAC=sinxcex8c, NAp=sin"sgr"p. In this case, the coherency factor "sgr" of the projection exposure apparatus shown in FIG. 1 is given as: "sgr"=NAC/NAp. 
To be more specific, the measure proposed in Japanese Patent Disclosure (Kokai) No. 7-153658 is directed to the projection exposure of a pattern of a photomask on a substrate by using a projection exposure apparatus. It is proposed that at least three imaging points are set within a predetermined range, which includes a focusing point, along the optical axis of the projection exposure apparatus relative to the same position on the substrate, and the pattern is projected under the condition that the coherency factor "sgr" is lowered in the imaging point present in the focusing point and/or in the vicinity of the focusing point, compared with the coherency factor "sgr" in the other imaging points. Also, where three imaging points are set, it is optional whether each imaging point is set equal to one third of the conventional one exposure dose, whether the coherency factor "sgr" in the central imaging point is made larger than that of each of the "sgr" edge imaging points, and whether the coherency factor a in the central imaging point is made smaller than that of each of the edge imaging points. In any case, in order to obtain the optimum exposure conditions in the multiple exposure method, it is necessary to change the coherency factor "sgr" of the illumination system for every exposure at different focuses. It is also necessary to control the exposure dose.
However, if it is intended to perform the change and the control described above in a scanning exposure, it is impossible to achieve the change and the control by employing the multiple exposure method in the scanning exposure apparatus disclosed in Japanese Patent Disclosure (Kokai) No. 11-214301. To be more specific, in the scanning exposure apparatus disclosed in this prior art, the scanning is performed with the mask and the wafer kept inclined relative to each other during the exposure within the slit restricting the exposure region. In this case, the amount of illumination within the slit is uniform. The coherency factor of the illumination system is also constant. Under the circumstances, it was impossible to control the coherency factor and the exposure dose of the illumination system for every focusing point, though such a control is required for optimizing the multiple exposure.
As described above, it is possible to carry out the multiple exposure by inclining the slit in the conventional step-and-scan system. However, it was difficult to control the coherency factor and the exposure dose of the illumination system for every focusing point, though such a control is required for optimizing the multiple exposure.
According to an aspect of the present invention, there is provided a scanning exposure method, in which, when a pattern formed on a mask is transferred onto a target substrate via an optical projection system, the projecting region of the mask is limited by a slit, and the slit is scanned relative to the mask and the target substrate so as to transfer the entire pattern region of the mask onto the substrate, wherein the exposure of the entire mask by the relative scanning of the slit is carried out at least twice by changing the exposure conditions so as to achieve a multiple exposure of the pattern image on the substrate in a superposed fashion, the focusing points in the multiple exposure are set in at least two ways, and at least one multiple exposure is carried out under the condition that the mask and the substrate are relatively tilted within the projecting region restricted by the slit.